Build your own radio telescope

​Astronomers use giant radio telescopes to observe black holes and distant galaxies. Why not build your own small-scale radio telescope and observe objects closer to home?

Detecting radio waves –
one of the giant antennas of
ALMA, the largest ground-
based astronomy project in
existence
Image courtesy of Iztok
Bončina / ALMA (ESO / NAOJ
/ NRAO)

When astronomers study the sky, they don’t just look at starlight. Stars, planets and nebulae shine across the whole electromagnetic spectrum, and the light that human eyes can see is only a narrow part of it.

Radio telescopes observe the sky for radiation at wavelengths that are thousands to millions of times longer than visible light. The huge antennas that scientists have built to observe these wavelengths have become icons of modern technology. Arecibo Observatory, so big it was built into a bowl-shaped valley in Puerto Rico, is instantly recognisable from the James Bond movie GoldenEye, while Jodrell Bank has dominated the skyline of Manchester, UK, for half a century.

The resolution of a telescope’s images depends both on the wavelength at which it operates and on the diameter of its dish. The longer the wavelength, the worse the resolution; and the larger the diameter, the better the resolution. Radio waves have a much longer wavelength than visible light, which is one reason why professional radio telescopes are enormous. Their huge size also helps them to capture the faint radiation from dim and distant objects. Nonetheless, the basic technology behind radio telescopes is quite simple and with some cheap equipment and simple tools, it’s quite easy to build a simple but functional one of your own.

I called my radio telescope design RYSIA (a girl’s name), or RadiowyY Śliczny Instrument Astronomiczny – Polish for ‘beautiful radio astronomy device’. With RYSIA, you can carry out simple observations of objects that radiate brightly in the radio spectrum. This includes the Sun, our own planet, and man-made communications satellites such as Hot Bird, Astra and Sirius.

Materials

Figure 1: Remove the mount
(A) at the back of the
satellite dish
Image courtesy of Boguslaw
Malański

Local scrapyards, shops selling second-hand TV equipment and online auction sites such as eBay are good places to buy the parts you need.

A satellite receiver dish

When you watch TV, the satellite dish focuses the transmissions from the satellite onto the receiver. In your radio telescope, the dish will serve a similar purpose: it will reflect the relatively faint radio waves onto the receiver. I recommend one with a diameter of at least 1 m. The dish can be of offset or parabolic type. A new dish costs approximately 100 zł (€24). A used dish should cost no more than 12-20 zł (€3-5). You can also search at scrap metal yards – this is where I found mine.

If the satellite dish has a mount, remove it (figure 1), as it just adds weight and makes it harder to move. The employee at the shop or scrap yard will probably be happy to remove it for you. Leave the arm attached, though.

A low noise block downconverter, or LNB

LNBs are an essential part of satellite TV receivers, and sit at the point where the dish focuses incoming rays. When we watch TV, the LNB receives and amplifies the signal, strips out unwanted frequencies and then converts the signal to a lower frequency. In your radio telescope, the LNB will be the receiver that detects the radio waves reflected by the dish. Any brand or model will work fine, even the cheapest. The price of a new downconverter is approximately 40 zł (€10), but they can be bought second-hand for a fraction of the price.

This tells us whether the LNB is receiving a signal, and if so, how strong it is. Be sure to get one that emits a sound when the antenna receives a strong signal; this makes it easier to demonstrate the device to large groups of students since everyone will be able to hear the signal. Additionally, the meter needs to be equipped with a dial or display that will allow you to measure the strength of the signal, so that you can make more precise measurements and compare different observations. Apart from these considerations, get the simplest and cheapest one you can find – the cost of this part is approximately 20 zł (€5).

Take the coaxial cable with only one connector, and strip the other end of the cable to reveal the metal core and the woven copper shield (figure 7).

Plug the connector on that coaxial cable into the satellite meter’s second socket (labelled ‘power’ or ‘receiver’; figure 8).

The other end of the cable now needs to be connected to your power source. Connect the woven copper shield to the battery’s negative terminal and the core to the battery’s positive terminal (figure 9).

Figure 7: The woven shield
can be seen folded back along
the cable. The metal core
projects from the cable
Image courtesy of Szymon
MalańskiFigure 9: Connecting the
power source
Image courtesy of Szymon
Malański

Figure 8: Powering the
satellite meter and receiving
the signal from the LNB
Image courtesy of Szymon
Malański

You have now built a basic, mobile radio telescope that is light and manoeuvrable enough to transport and point at different objects by hand.

Connecting your radio
telescope. A: the LNB, on
which the radio waves will
be focused; B: the satellite
signal meter; C: the power
supply
Image courtesy of Szymon
Malański

If you wish to build a mounted device, you will need to attach it to an object (such as a heavy tripod) that allows you to adjust both the azimuth (the horizontal direction that the telescope is pointing in) and the altitude (how high or low it is angled).

Activities using your radio telescope

You now have a radio telescope that works on some of the same principles as the gigantic radio telescopes that are used to investigate the earliest days of the Universe, capturing radiation from very distant galaxies (see Mignone & Pierce-Price, 2010). Although your much smaller telescope cannot detect distant stars, you can use it to demonstrate to your students that the Sun and other objects radiate not only visible light but also radio waves. Furthermore, you can find the position of the Sun on a cloudy day, demonstrate that the surface of Earth emits radio waves, and locate satellites.

If you used a parabolic antenna to build your radio telescope, you will need to point its axis directly at the object you are observing. If you used an offset antenna, however, you must take into account the angle by which it is offset. Most manufacturers do not provide this parameter but it can easily be calculated (this can be an additional task for the students). In practice, the arm of the satellite dish on which the LNB is mounted indicates the direction from which the signal is received (figure 10).

Figure 10. a) A front view of the two types of satellite dish: parabolic (1) and offset (2) dishes, showing the position of the LNB (3)
b) Cross-sections of an offset satellite dish (left) and a parabolic satellite dish (right), showing the angle of elevation (4)
Image courtesy of Szymon Malański

Observing the Sun

Using RYSIA on a cloudy day
to locate the Sun
Image courtesy of Szymon
Malański

The Sun emits radiation across much of the electromagnetic spectrum. On a clear day, try pointing your radio telescope at the Sun and at a patch of empty sky. Compare the readings. Repeat the experiment on a cloudy day; the Sun’s location can easily be determined, despite the clouds. Ask your students why they think that visible light is blocked by the clouds but radio waves can penetrate.

You could also ask your students how they can distinguish the Sun’s radiation from a satellite signal, particularly as they sometimes appear close together in the sky. The answer: the satellite signal is polarised (horizontally or vertically) whereas radiation from the Sun is not. So if you rotate the radio telescope dish and the signal strength is unchanged, the signal is coming from the Sun.

Observing Earth

Objects around us, including buildings, plants, people and even the ground under our feet, emit radio waves, reflected from the Sun or Earth. Try comparing readings for different objects. Thanks to the auditory signal from the satellite signal meter, you should be able to detect the location of buildings and trees around you easily, even when blindfolded. To make sure that the signal does not come from the Sun itself, make sure you carry out these experiments by pointing the dish away from the Sun.

Detecting heat

Most astronomical phenomena produce electromagnetic radiation because they are hot. The higher their temperature, the shorter the wavelength they can produce. At around 5500 ºC, the Sun produces plenty of visible light as well as infrared and radio waves. Colder objects have to be detected using infrared or radio telescopes. You can demonstrate this by pointing your radio telescope at a hotplate as it heats up. It will only begin to emit visible light at around 700 ºC, but your telescope will detect the radio waves emitted well before that.

Be aware that during the spring and autumn equinoxes, the Sun shines above the equator and can interfere with satellite reception when the Sun and the satellite are in the same area of sky. Wolfram Alpha has a map of the Sun’s location relative to a satellite, so this is easy to avoid.

Figure 11: In summer, the Sun is above the line representing the location of geostationary satellites. In winter, it is below this line
Image courtesy of Szymon Malański

The European Southern Observatory (ESO) is one of the partners in the radio observatory ALMA, the Atacama Large Millimeter/submillimeter Array, an ensemble of huge, high-precision antennas on the Chajnantor plateau in Chile’s Atacama region. Also on Chajnantor plateau is ESO’s radio telescope APEX.

Author

Boguslaw Malański is a physics and astronomy teacher. He gained a physics degree and a PhD from the University of Łódź, Poland, after which he spent nine years as a physics lecturer at the University of the North-West in South Africa. He was also involved there in performing science experiments for local schools together with Pastor Bernd-Peter Jensen. Boguslaw is currently employed at the local planetarium and astronomical observatory in Łódź, where he teaches astronomy and physics. He also runs a small, non-profit experimentarium for local schoolsw5.

Szymon Malański, Boguslaw’s son, is a 5th-year student studying telecommunications and computer science at the Technical University of Łódź, Poland. He enjoys experimenting and is keen on classical analogue photography.

Comments

I use a satellite finder with audio output. Connecting the audio to the input of the sound card and use software like Audacity one can record the signal. Once you have the digital signal one can play with it and also use Sonic Visualizer to manipulate the signal and get spectrum etc ....

This is an excellent article. It doesn't delve into the non-trivial issues of system noise temperature, gain stability, measurement bandwidth issues et al, but that's good. It's a brilliant, practical introduction which will inspire students to learn more.

Recommended is 1m for the dish. I'm wondering how critical this measurement is. I know the wider the diameter the better but for Sun, satellites would the more common 60cm or 80cm satellite dish sizes suffice?

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If you would like your students to discover that the Sun or a hotplate emits a lot more than just visible light, that visible light is blocked by the clouds but radio waves are not, or that the electromagnetic spectrum consists of a variety of very interesting radiation; if you want your students to be able to find the position of the Sun on a cloudy day or to locate geostationary satellites; if you would like them to distinguish polarised radiation from non-polarised radiation; if you want to use a radio telescope in your lessons, constructed by your students and capable of helping you to teach all these topics and much more, then you will definitely be interested in the ideas presented in this article.

Using cheap materials and easy-to-follow instructions, you can build a simple but functional small-scale radio telescope. The activities suggested in this article are both interesting and applicable to a range of scientific topics (e.g. orbits, light, radiation and its effects on the body, and the electromagnetic spectrum), which can be covered in physics, astronomy and biology lessons.